Clone 5-329 recognizes CD74, a non-polymorphic type II integral membrane protein. CD74 is also known as MHC class II–associated invariant chain and exists in multiple isoforms in human ranging from 33 to 43 kDa in size. Initially known for its role as the MHC II chaperone, CD74 primary function is suggested to regulate the MHC class II molecules folding and intracellular sorting. Recent studies however suggest further roles of CD74. It interacts with cytokine-like molecule called macrophage migration- inhibitory factor (MIF) and induce ERK1 and ERK2 signaling in a CD44 dependent manner. CD74 is also shown to form complexes with CXCR2 and CXCR4. A direct role of CD74 in the maturation of follicular B cells and accumulation of marginal zone B cells has also been suggested. CD74 was further shown to be one of the host molecules utilized by

to successfully attach to the gastric epithelial cells. Expression of CD74 is found on B cells, dendritic cells, epithelial cells, subsets of activated T cells, and low level expression is found on monocytes and macrophages.